Preparation of solutions of hydrofluosilicic acid



July 16, 1929. w. MLLER ET A1.

PREPARATION 0F SOLUTIONS OF HYDROFLUOSILICIC ACID Filed De@ 22, 1926 .lI 1 v ,f u. l .|I Il I- ||l`l|| Il' Illbl illy? f w fr u F u n u u It LL v .r

bmbf@ MQEODJL Patented July 16, 1 929.l

1,720,965 lmural) STATES- PATENT OFFICE..

mam uLLna, .or waNTonr-anmnax n/Hauauac, AND wrm-:amr miriam, ormarianne, cnam?. 1.

' PREPARATION 0F 'SOLUTIONS 0F HYDQOFLUOSILICIC AGH).

Application led December 22, 19.88, Serial No. 156,512, and in GermanyDecember 28, 1925.

'This invention relates to the manufacture of a highly concentratedsolution of hydrofluosilicic acid. Hydrofluosilicic acid is alreadymanufactured on an industrial sca-le 5 by generating silicon fluoridefrom a mixture of sand and f'luor spar by the action of sulphuric acid,and by bringing this silicon fluoride in contact with water, variousapparatus having already been proposed for carrying outl this completereaction without cessation. As is well known, it is impossible `to workin practice by introducing silicon fluoride directly into water, as insuch a case the admission pipe is easily liable to be choked by thecolloidal silicio acid separated. It has been also attempted to meetAthese difiiculties by bringing gases containing siliconfluoride` incontact with atomized 'water or steam. Finally, thin water mist wasproduced by means of rotating discs. and the silicon fluoride was passedthrough it. All these processes for the decomposition of siliconfluoride fail, owing to the reaction between silicon fluoride and waternot being instantaneous, but taking some time. which is probably due tothe formation of colloidal silicio acid, and also owing to the colloidalsilicio acid which settles on the parts of the apparatus. being a veryinconvenient addition to the aqueous solution of hydrofluosilicio acidproduced.

In addition to the above, the reaction between water and siliconfluoride takes place at the ordinary temperature. Contrary to publishedstatements. it ceases at a higher temperature. This behaviour explainsthe fact that silicon fluoride escapes with the other gases from theopening-up vessel of the superphosphate factories. in spite of thepresence of considerable quantities of steam. This observation has been,moreover. confirmed by the applicants who found that when watertrickling towers were used for the manufacture of hydrofluosilicic acid,the reaction in the first tower continued only as lona as it was stillcold, and ceased as soon as the tower was heated by the hot gases to acert-ain extent. The reaction then took place in the second tower untilit also became too hot, and so on. But the siliconfluoridecontainingpgases which are used for the absorption and for thereaction` are always of a fairly high temperature, whether they be wastegasesfrom the opening up of superphosphate vessel, or whether theyA be0btained directly from a prepared mixture of v fluor spar, sand andsulphuric acid.

The applicants have discovered that the most suitable. pparatus orinstallationsfor obtaining realy high concentrations ofhydrofluosi'licica id, are those in which the gas is caused to pass along surfaces coatedwith thin layers of water, which are of such a nature that they arealways kept free from the silicio' acid separated, and therefore alwaysact as heat discharging surfaces. Such apparatus which are suitable forthe purpose, are for instance the apparatus used for gas washing and thelike, which comprise substantially a closed tube and a spindle providedwith discs and rotating in the said tube. the discs of which dip withtheir lower part into the aqueous liquid which is to absorb thehydrofluosilicic acid. The

slowly rotating discs whichas far as possible fill up the wholeliquid-free cross section of the tube, by means of their wetsurface.give the silicon fluoride an opportunity. and also suflicient time. forthe reaction. and the silicic acid separated during this time. is washedaway from the dippingy disc half, so that the portion emerging againfrom the liquid. is free from silicio acid. and offers a fresh reactionsurface to the silicon fluoride.

It is preferable to proceed by connecting to the lower face of the tubea large tank in open communication with it. in order to give the silicioacid which represents a considerable volume. an opportunityT to settlein a space free from movements of liquid. Several such decompositionunits for the reaction between Water and silicon fluoride may preferablybe used simultaneously with the necessary change-over parts in order toenable them to be placed in or out of position in any desired sequence.according to requirements. Such an installation may preferably beprovided withy filter devices, in order first to separate the formeddilute solutions of hydrofluosilicic acid from the -silicic acidseparated, and to use these solutions again as aqueous liquid for thedecomposition of further quantities of silicon fluoride.

Such an installation for the decomposition of silicon flouride by water,complies with the three necessary conditions, i. e. of constantlykeeping cool the reaction surfaces, and of giving the reaction surfacesthe form of surfaces provided with a thin water coating, and finally ofkeeping the reaction surfaces free from separated silicio acid.

The single discs may be of different character. They may bein verticalposition relatively to the axis) of rotation, and provided with slots.They may be constituted also, in order to avoid an unnecessary increaseof the passage resistance of the material to be treated, by severalportions obliquely mounted on the spindle like fan blades, and theseportions may further be bent or curved. The essential point is thatlarge moist surfaces should be provided in order to carry the reaction,which takes time, through` to the end.

An example of an apparatus which may be used to practice the presentinvention is illustrated in the drawings in which Fig. 1 is an end viewyat the left of Fig. 2. looking in the direction of the arrow, and Fig.2 is a side view. The apparatus comprises a fluid container A, theenclosing cover B, the horizontal rotatable shaft D and the discs Cattached to the shaft D. The discs may be of any desired form `asexplained above. The inlet for the silicon fiuoride containing gases isindicated at E, and the outlet for the residual gases at F. The fluidinthe container A extends to the discs D so that their parts or wingswill be moistened as they rotate with the shaft D.

Having described our invention, claim l. The process of producinghydrofluosilicic acid which comprises contacting silicon fluoride withthe surface of a thin film of an aqueous medium, thereafter replacingsaid film by another aqueous film capable of fur'- ther reacting withsilicon fluoride, and repeating the aforesaid surface contact.

2. The process of producing hydrofiuosilicic acid which comprisescontacting silicon fluoride with a surface containing a thin film ofwater, removing said film after reaction with the silicon fluoridetreplacing it by means of another film and repeating the aforesaidcontact.

3. The process of producing hydrofluosilicie acid which comprisesContactin silicon fluoride with surfaces coated with t in layers of anaqueousv medium and thereafter washing away the silicio acid andhydrofiuosilicic acid formed during such contact by means of an aqueousmedium and thereby simultaneously renewing said aqueous layers. f v

4. The processof producing hydrofluosilicic acid which comprisesproducing a thin film from a portion of a body of water, bringingsilicon fluoride in surface contact with said film, replacing said filmby another formed from another portion of said Water, adding thepreceding film to said body of water and continuing this operation untilthe desired concentration of hydrofiuosilicic acid is attained.

5. The process of producing hydrofluosilicic acid which comprisescontacting silicon fiuoride with a surface wetted with an aqueousmedium, removing reaction products from said surface and wetting saidsurface anew for further contact with silicon fiuoride.

6. The process of producing hydrofiuosilicic acid which comy risescontacting silicon fluoride with a sur ace wetted with water, therebyproducing silicio acid and hydrofiuosilicic acid, removing the silicioacid and hydroiiuosilicic acid so formed, wetting the said surface againand continuing the wetting and removal until the desired amount ofhydroiiuosilicic acid is produced.

7. The process of producing hydrofiuosilicic acid which comprisesbringing silicon fluoride and water into chemical reactive relationshipand maintaining the reacting substances throughout the process atsubstan` tially ordinary temperature to promote the formation of thehydrofluosilicic acid.

In testimony whereof we have signed our names to this specification.

WILHELM MLLER. WILHELM KRETH.

till

